Certain cells in the body respond not only to chemical signals, but also to ions such as extracellular calcium ions (Ca2+). Changes in the concentration of extracellular Ca2+ (referred to herein as “[Ca2+]”) alter the functional responses of these cells. One such specialized cell is the parathyroid cell which secretes parathyroid hormone (PTH). PTH is the principal endocrine factor regulating Ca2+ homeostasis in the blood and extracellular fluids.
PTH, by acting on bone and kidney cells, increases the level of Ca2+ in the blood. This increase in [Ca2+] then acts as a negative feedback signal, depressing PTH secretion. The reciprocal relationship between [Ca2+] and PTH secretion forms the essential mechanism maintaining bodily Ca2+ homeostasis.
Extracellular Ca2+ acts directly on parathyroid cells to regulate PTH secretion. The existence of a parathyroid cell surface protein which detects changes in [Ca2+] has been confirmed. Brown et al., 366 Nature 574, 1993. In parathyroid cells, this protein acts as a receptor for extracellular Ca2+ (“the calcium receptor”), and detects changes in [Ca2+] and to initiate a functional cellular response, PTH secretion.
Extracellular Ca2+ can exert effects on different cell functions, reviewed in Nemeth, et al., 11 Cell Calcium 319, 1990. The role of extracellular Ca2+ in parafolicular (C cells) and parathyroid cells is discussed in Nemeth, 11 Cell Calcium 323, 1990. These cells have been shown to express similar Ca2+ receptor. Brown et al., 366 Nature 574, 1993; Mithal et al., 9 Suppl. 1 J. Bone and Mineral Res. s282, 1994; Rogers et al., 9 Suppl. 1 J. Bone and Mineral Res. s409 1994; Garrett et al., 9 Suppl. 1 J. Bone and Mineral Res. s409, 1994. The role of extracellular Ca2+ on bone osteoclasts is discussed by Zaidi, 10 Bioscience Reports 493, 1990. in addition keratinocytes, juxtaglomerular cells, trophoblasts, pancreatic beta cells and fat/adipose cells all respond to increases in extracellular calcium which likely reflects activation of calcium receptors of these cells.
The ability of various compounds to mimic extracellular Ca2+ in vitro is discussed by Nemeth et al., (spermine and spermidine) in “Calcium-Binding Proteins in Health and Disease”, 1987, Academic Press, Inc., pp. 33-35; Brown et al., (e.g., neomycin) 128 Endocrinology 3047, 1991; Chen et al., (diltiazem and its analog, TA-3090) 5 J. Bone and Mineral Res. 581, 1990; and Zaidi et al., (verapamil) 167 Biochem. Biophys. Res. Commun. 807, 1990. Nemeth et al., PCT/US93/01642, International Publication Number WO 94/18959, Nemeth et al., PCT/US92/07175, International Publication Number WO 93/04373, Nemeth et al., PCT/US94/12117, International Publication Number WO 95/11221 and Nemeth et al., PCT/US95/13704, International Publication Number WO 96/12697 describe various compounds which can modulate the effect of an inorganic ion on a cell having an inorganic ion receptor, preferably modulate the effects of calcium on a calcium receptor.
The object of the present invention is to provide a novel inorganic ion receptor active compound having the structure different from the compounds described above.